1. Field of the Invention
The present invention relates to a technique to etch an organic film, which constitutes part of a multi-layered resist, into a mask pattern through which the objective film is etched.
2. Description of the Related Art
In the manufacture of semiconductor devices, metal interconnects formed in inter-layer insulation films made of, for example, oxide silicon (SiO2) are becoming finer and finer. To cope with this trend toward finer interconnects, a three-layer resist is formed in some cases. The three layers are, for example, an organic film, a SiO2 film by the SOG (Spin On Glass) method, and a photoresist film which are formed in this order on the insulation film.
Specifically, interconnects are formed by using this three-layer resist as described here. After the photoresist film is formed into a resist pattern by photolithography, the SiO2 film is etched through this resist pattern to form a mask pattern having side walls inclined so that the apertures narrow with depth. Then, after the lower organic film is etched according to the mask pattern to form the organic film into a mask pattern, the insulation film is etched according to the patterns formed from the respective layers. Consequently, recesses are formed as contact holes and via holes where fine interconnect lines, narrower than those of the resist pattern, will be buried. Conventionally, O2 gas in the plasma state is used to etch the organic film.
In order to further improve semiconductor devices in operation speed, it is demanded to lower the relative dielectric constants of interlayer insulation films. Instead of conventionally used SiO2, a lower dielectric constant porous film which contains silicon, carbon, oxygen, and hydrogen (SiCOH film) is sometimes used as an interlayer insulation film. However, this SiCOH film is liable to be damaged if exposed to oxygen plasma. In addition, if plasma O2 gas is used to etch the organic film, the organic film is etched isotropically. In other words, since etching by generated oxygen (O) radicals is active not only downward but also laterally, the organic film has such a bowing profile that each sidewall of the pattern, when viewed in vertical section, is laterally widened like a bow, resulting in a mask pattern with poor perpendicularity sidewalls. If the mask pattern has such a bowing profile, it may be impossible to form sufficiently narrow interconnect lines in the insulation film since the insulation film is etched wider.
Accordingly, the present inventors made an accumulative study of the etching gas and contrived the use of CO2 gas plasma to etch the organic film. Use of CO2 gas plasma is effective since it can not only suppress damage to the SiCOH film but also reduce the generation of oxygen radicals as compared with O2 gas plasma and consequently suppress isotropic etching of the organic film. Since the mask pattern formed from the organic film does not have a considerable bowing profile, it is possible to accurately control the etching width when the organic film and the lower insulation film is etched. With the progress of interconnects in minuteness, however, etching of the organic film is demanded to meet still higher perpendicularity requirements. In the use of CO2 gas plasma it is becoming difficult to suppress the occurrence of bowing profile sufficiently enough to meet this demand.
Etching of low dielectric constant films by use of a gas composed of H2 and CO2 is disclosed in JP-A-2002-16050. Unlike the present invention, however, there is no mention of etching an organic film of a multi-layered resist in forming a mask.